Energy quality factor and exergy destruction processes analysis for a proposed polygeneration system coproducing semicoke, coal gas, tar and power

A new polygeneration system was established by combining coal pyrolysis with semicoke combustion, semicoke gasification and a steam Rankine cycle, and calculation approaches were developed to analyze the system. The energy and exergy efficiencies of the system were 68.3% and 76.8%, respectively. The standard energy quality factor αθ values of the 17 components and the energy quality factor α of each system stream were obtained based on the benchmark of the selected environmental reference state. The features of the energy grade of the components were elucidated according to their energy quality factors. And the exergy destruction processes in system blocks were exhibited and analyzed in combination with the concept of the energy quality factor. In the DRYER and PYRO, the majority of exergy destructions were used to pay the thermodynamic penalty of the increased energy grade of the coal. In the BUR and GASIF, the energy grade difference between the semicoke and output gas was the main cause of exergy destruction. Especially in the BUR, the energy grade difference was enormous and the exergy destruction was large. In HEX2, the average energy grade difference between QED and QEA was the main cause of the exergy destruction. Finally, from the perspective of the energy quality factor, some potential improvements were analyzed to reduce the exergy destruction in the blocks.